The main requirements of a radio transceiving apparatus, to be installed on board mobiles, such as aircrafts, small boats, motor vehicles, are low cost and small size.
To satisfy such requirements, the used antennas, being simple and fixed, have low directivity and hence they receive, in addition to the main signal directly arriving from the transmitter or from the repeater, multipath signal reflected by the earth surface or other various fixed or mobile obstacles.
Since the antenna is installed on board a mobile, or owing also to the same earth surface motion (e.g. seawaves), in certain instantaneous positions of the mobile, undesired reflected signals sum up together degrading the main signal and giving rise to a resultant signal whose level, for short time intervals, can drop below the correct-reception threshold of the receiver thus originating a burst noise in the decoded signal. This effect is known in the art as "multipath fading".
Coding devices known in the art as vocoders are usually used for speech-signal transmission on low bit-rate channels (up to 4.8 kbit/s).
Said devices analyze the speech signal at intervals, whose typical length is nearly 20 ms, extracting for each interval a set of parameters which is then transmitted at low bit-rate, usually at 2.4 kbit/s. In reception on the contrary such devices use received and decoded sets of parameters and reconstruct the original speech-signal.
According to typical "fading" duration (99% probability that it is shorter than 22.5 ms), the mobile vocoder receiver can receive one set of erroneous parameters, if the burst noise falls within an interval, or two consecutive sets of erroneous parameters, if burst noise spans two consecutive intervals.
To correct the erroneous parameters, two known techniques could be used.
The first known technique uses a self-correcting code for encoding the parameters in transmission.
Such codes however introduce high redundancy in transmitted data, i.e. a high transmitted-data/actual-data ratio.
In this case, to use the same vocoder (e.g. 2.4 kbit/s Vocoder), transmission bit-rate ought to be greatly increased, what cannot be achieved without reaching or furthermore exceeding maximum bandwidth available to each transmission channel. In the alternative, transmission bit-rate could be kept standard (e.g. 2.4 kbit/s), by using a lower bit-rate vocoder, which however would be much more complex and expensive, and of a lower transmission quality. Hence such a vocoder could not be used in our case.
The second known technique, described e.g. in the paper entitled "Vocoder speech through fading channels", B. Gold., J. Lynch, J. Tierney, ICASSP '83 conference, held in Boston, consists of requesting retransmission of parameters taken as erroneous by the receiver.
According to this technique, during the whole retransmission time, the vocoder of the mobile receiver generates no output signal, and recovers time spent waiting for correct parameters by speeding up the speech synthesis process in the subsequent intervals, thus introducing an artificial time compression of the reproduced speech.
This second method cannot be applied in geostationary satellite communications since, as the total path covered by transmitted waves is long, the delay introduced in case of retransmission would be too high. In fact the mean delay for a geostationary satellite communication is 270 ms; each retransmission would take at least 540 ms; for a 2.4 kbit/s vocoder that would mean 24 consecutive silence intervals, to be afterwards recovered, with consequent excessive degradation of speech reproduction quality.
In direct transmission systems, on the contrary, this technique is theoretically applicable, as the path covered by the transmitted waves is short and then the delay introduced in case of retransmission is not considerable. Yet from the implementation standpoint, considerable complications would be introduced in the structure of receiving and transmitting apparatus.
In addition the retransmission technique could not obviously be applied in the case in which the apparatus installed on board the mobile would comprise the only receiving side.
These problems are solved by the present invention of a method of and a device for correcting burst errors on low bit-rate coded speech signals, transmitted on radio communication channels, which does not require retransmission in case of error and does not utilize self-correcting codes for data encoding, but detects in the mobile receiver the presence of transmission errors due to burst noise by distance measurements between the values assumed by speech parameters in consecutive intervals, and, in case of error, substitutes parameters obtained with linear interpolation techniques for the parameters taken as erroneous.